This invention relates to pulse width modulation techniques and, more particularly, to a method of and apparatus for converting a plural-bit digital signal to a corresponding pulse width modulated signal, the full duration of the pulse width being subdivided such that, if the pulse width modulated signal is smoothed to derive an analog level, the resultant analog level exhibits minimal ripple.
Pulse width modulating systems have been used for various applications. In a typical application, a pulse width modulator is used to convert, for example, an encoded digital signal into a corresponding pulse width modulated signal, and this pulse width modulated signal then can be used to produce an analog level corresponding to the encoded digital signal. For example, if the pulse width modulated signal is a recurrent signal, it can be filtered by a low-pass filter so that, at each recurrence thereof, a corresponding analog level is produced. Alternatively, the recurrent pulse width modulated signal can be transmitted through an integrator to derive the aforementioned corresponding analog level.
A pulse width modulator that can be used in a digital-to-analog conversion system of the type mentioned above finds ready application in an electronic tuner, such as a tuner used to select a particular channel in a television receiver or to select a particular station in, for example, an FM radio receiver. In this type of electronic tuner, a voltage-controlled variable reactance device, such as a varactor or variable capacitance diode, is employed as the tuning element, and the control voltage for the variable reactance device is derived from a digital signal representing the channel or station to which the tuner is to be tuned. A pulse width modulator can be used to so derive the analog control voltage from the digital signal.
In the typical pulse width modulator wherein a single variable-width pulse is produced during a predetermined period, this pulse generally being recurrent in successive periods, the relatively long intervals between pulses result in a significant ripple factor in the filtered analog voltage. When used in an electronic tuner of the type described above, this ripple factor can lead to an erroneous setting of the variable reactance device and, consequently, an improper tuning of the electronic tuner. If this ripple factor is reduced by, for example, increasing the filtering of the pulse width modulated signal, the effective time constant of the filter correspondingly is increased, thereby imparting a substantial delay between a change in the pulse width modulated signal and a corresponding change in the analog signal level. This delay also is a source of erroneous tuning of the electronic tuner.
It had been thought that both the ripple factor and time delay of the aforementioned prior art digital-to-analog converters using a pulse width modulator could be minimized if the frequency of the recurrent pulse width modulated signal is increased. However, if the frequency is increased, it follows that the period during which the pulse width is produced must decrease. This, in turn, limits the maximum width of the pulse width modulated signal; thereby limiting the range and sensitivity thereof. That is, the change in the width of the pulse in response to each incremental bit in the digital signal must be limited if the pulse width modulation frequency is increased. With this limitation, it becomes very difficult to discriminate accurately between slightly different pulse widths. Thus, an erroneous analog level may be produced, resulting in an erroneous tuning of the aforementioned electronic tuner.